Rock art recording and digital enhancement

Detailed field recordings are being made of thousands of rock images.  Digital enhancement of the rock art from selected sites is to be undertaken using high-resolution photographs so as to reveal faded motifs and older images hidden by overlying paintings.

How we do it

Following a close visual inspection of an art panel and making notes on what superimpositions can be observed, it is essential to capture an adequate photograph of the panel.  In many cases, large panels cannot be photographed in a single image due to either a lack of distance between the ceiling panel and the floor, or lack of any vantage point to photograph the wall.  This problem can be overcome by either:

  • taking overlapping photographs at right angles to the panel and stitching the individual photographs into a single composite image (photomosaic) using appropriate software; or
  • using a “fish-eye” lens and an appropriate programme to remove the distortion.

The resultant image is then imported into a graphics programme that utilises layers, such as Adobe Photoshop®.  This enables the isolation of motifs from different layers of the superimposition onto their own layer “sheet”, either through drawing (outlining or detailed tracing), or selective capture using the magic wand feature.  This process is relatively easy with the upper and most recent layers, where all or most of a motif is readily visible.

In the case of lower layers, motifs can be enhanced using image processing software optimised for pictographs such as D-Stretch®, which selectively highlights colours depending on the enhancing combination (or colour space) used.  The process undertaken by D-Stretch® is an image processing technique frequently used with multi-spectral satellite imagery; the de-correlation of colour bands via transformation into alternative colour spaces, and then performing a contrast ‘stretch’ or enhancement to highlight the differences.  The process is very similar to a Principal Component Analysis that can be applied to many data sets where there may be a relationship between one variable and another, including bands of light intensity values found in digital images. This image transformation can be achieved with advanced image processing and analysis software, but the image manipulations in DStretch® have been optimised by a rock art researcher as being the most useful for analysing pictograms.   While not replacing the more manual methods previously used, as the D-Stretch® colour spaces are standardized, the resultant images can be readily repeated by other researchers.  Also, while only rarely revealing “invisible” motifs, D-Stretch® makes the visualising of very faint images much clearer and in a number of cases where only traces of pigment can be seen on the original photograph, D-Stretch® will permit the form of the motif to be defined.  D-Stretch® operates as a plug-in for ImageJ, a public domain, Java-based, image processing and analysis program freely available through the National Institutes of Health (USA) [http://rsb.info.nih.gov/ij/index.html.

Using software like Photoshop® that incorporate multiple layers within a single file, if the superimpositions are simply that of one colour layer over another, each colour can be given its own layer sheet and when the visual analysis has been completed, the layers can be printed separately, motifs numbered and described, and the layers flattened to give a drawing of the mosaic.  If the superimpositions are complex with different colours or colour combinations represented in each layer, then, starting from the upper layer, each layer of motifs is given a layer sheet and the above process repeated.  The ability to keep the interpreted layers of superimposition separate also gives an opportunity for greater analysis of regional and chronological style.

To produce the final interpretation of the underlying layers, where possible the fragments of each motif are joined and filled with a less intense colour of the pigment so that the interpretation can be visually appreciated.  In many instances, however, the full interpretation of the underlying motifs will not be possible due to their high degree of fragmentation.

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